TIMING AND MECHANISMS OF TERMINATION OF THE INTERIOR OF A SHALLOW MARINE CARBONATE PLATFORM: THE PINGGUO PLATFORM, NANPANJIANG BASIN, GUANGXI PROVINCE, SOUTH CHINA

The Pingguo platform is one of several isolated Triassic platforms in the Nanpanjiang Basin, a rapidly subsiding embayment in the Yangtze microcontinent of the equatorial eastern Tethys. The Pingguo platform evolved from a low-relief bank followed by vast areas of platform interior termination (step back) and development of smaller pinnacle platforms. Three sections were measured through the termination interval in a NW cross-section. The vertical facies pattern consists of: 1) molluscan dolowackestone-packstone with microbial laminites and fenestrae, 2) an interval with numerous mineralized and bored hardgrounds, 3) thick-bedded, dark-gray molluscan and oncolitic wackestone-mudstone, followed by 4) dark, organic-rich, nodular-bedded, and laminated lime mudstones containing echinoderms, ammonoids, conodonts, and radiolarians, and 5) shale. The molluscan dolowackestone-packstone occurs in the uppermost shallow-subtidal portion of the carbonate platform and contains features consistent with restricted subtidal to intertidal conditions. Deepening and "stalling" of carbonate productivity are indicated by the hardgrounds in the overlying facies and the dark gray color of the molluscan and oncolitic wackestone-mudstone which may have been deposited between normal and storm wave-base. Shifting to deep, sub-photic conditions, shut down of carbonate productivity, and termination of the platform interior are indicated by the sedimentary structures and pelagic fossils in the nodular-bedded, and laminated lime mudstone. Termination of the interior occurred during the Early Anisian (Aegean) as indicated by the conodonts Neospathodus abruptus, Neogondolella regale, and the ammonoids Bulogites, and Paraceratites. The facies succession indicates the platform was terminated by drowning below the photic zone by a combination of subsidence and sea level rise. In addition siliciclastic flux contributed to termination as evidenced by increasing magnetic susceptibility (detrital magnetite) in the nodular-bedded, and laminated lime mudstone. Nodular-bedded and laminated lime mudstones of the termination interval are relatively argillaceous resulting from shut down in carbonate productivity and concentration of background siliciclastics. Documentation of the interior termination and bank to pinnacle architecture of the Pingguo platform is important for understanding regional patterns in differential tectonic subsidence and platform evolution across the basin. Biostratigraphic correlation and radioisotope age dates were used to constrain comparisons of coeval platform events. The Pingguo platform, in the middle of the basin, and was terminated later than the southernmost platform but earlier than northerly platforms. Thus, comparison of timing of termination of the platforms demonstrates a south to north increase in platform longevity with earlier termination of the southernmost platforms. This is explained by higher rates and earlier onset of foreland basin subsidence in the southern area of the basin and later onset of foreland subsidence in the north.